1. Field of the Invention
The present invention relates to an image forming apparatus using powder toner.
2. Description of the Prior Art
It is known to employ an image forming method using an ink-jet printer for word processors, facsimile machines, computers, and the like.
The ink-jet printer is a typical example of a nonimpact printer, where pressure is applied to a prescribed liquid ink while a piezoelectric element or the like applies ultrasonic vibration thereto, so that the liquid ink is spurted from an ink nozzle into a prescribed electric field. The ink particles are controlled by the electric field and made to adhere to a recording sheet to form an image thereon. Such an ink-jet printing has the advantage of being able to form a clear image without generating noise during the formation of the image. On the other hand, this method is disadvantageous in that it requires the use of a special kind of recording sheet with its surface appropriately treated so as to control the rate at which the ink filters into the recording sheet. Further, the nozzle through which the ink is supplied tends to become clogged with foreign substances or the like contained in the ink.
To overcome the above difficulties with the ink-jet printer, Japanese Laid-Open Patent Publication No. 60-263962, for example, discloses an image forming apparatus using powder toner as an image forming medium. This image forming apparatus is equipped with a toner control means which controls the passing of toner particles through a toner passage in the form of a pinhole by an electrostatic attraction generated in accordance with image output signals. Through this toner control means, the toner particles are selectively fed onto a recording sheet, thereby forming a prescribed image on the recording sheet.
In such an image forming apparatus, a sheet of plane paper without surface treatment can be used as the recording sheet. Further, since the powder toner in the form of fine particles is used, the toner passage is prevented from becoming clogged with the toner.
The toner control means comprises an insulating substrate with a number of pinhole-shaped toner passages formed therethrough and a pair of electrodes sandwiching the insulating substrate so that an electric field is formed inside each of the toner passages. The pair of electrodes also have holes extending therethrough, respectively, each having the same size as that of the toner passage. The electrodes are disposed on the opposite surfaces of the insulating substrate, respectively, in such a manner that each hole is aligned with the corresponding toner passage. A prescribed voltage is applied between the pair of electrodes to form an electric field in a prescribed direction inside each toner passage, thereby allowing toner particles to pass through the toner passage. On the contrary, when it is required for toner particles not to pass through the toner passage, an electric field in the direction opposite to the above direction can be formed by applying a prescribed voltage between the electrodes.
A base electrode is disposed downstream in the toner passing direction with respect to the toner control means (on the recording sheet supply side). The recording sheet is placed on the base electrode. A prescribed potential difference is produced between the base electrode and the electrode disposed on the toner feed side of the toner control means to form an electric field therebetween, so as to ensure that the toner moves toward the recording sheet.
In the image forming apparatus as described above, a dot is formed by the toner which has passed through each toner passage, and a group of dots form an image such as a letter. The toner passages are generally disposed in a line perpendicular to the direction of the conveyance of the recording sheet. A voltage signal as a control signal is input to the pair of electrodes of the toner control means to control the formation of the image.
In the above image forming apparatus, a fixed amount of toner is fed onto one electrode disposed on the insulating substrate by means of a toner feed roller. The toner feed roller may be in contact with or in close proximity to the electrode. Thus, the toner is present on the electrode not only when an image is formed but also when an image is not formed. As a result, the toner may enter the toner passages when an image is not formed, thereby causing clogging of the toner passages.
The objective of this invention is to solve the above-described prior art problem and to provide an image forming apparatus in which toner does not enter into the toner passages when an image is not formed so as to prevent the toner passages from clogging with the toner.